Such systems employ an oil pump, driven by the engine, for ensuring sufficient circulation of oil. While a deprivation of oil can result in damage to engine components, over-pressurisation is also undesirable. For example, too high an oil pressure can destroy paper filter elements. To prevent over-pressurisation, lubrication systems are provided with pressure relief valves. These valves vent oil back into the oil sump or the pump's suction side when the oil pressure reaches a pre-set limit.
One known type of valve is described in U.S. Pat. No. 6,116,272. Therein, a pressure relief valve is integrated into the oil pump. The valve has a piston positioned within a cylindrical bore which is biased in one direction by a spring. Pressurized oil from the oil pump outlet forces the piston against the spring. As the oil pressure increases, the piston moves within the bore to a point where vents on the edges of the piston allow oil to escape back to the sump or pump.
With this type of conventional valve, the pump outlet pressure is used to determine when the valve will open to internally by-pass the excess oil from the discharge side back to the suction side. A typical valve of this type would be set to open at 7 bar.
Although such a conventional valve is simple and reliable, the pump's power consumption is relatively high as it always has to work to increase the pressure from atmospheric to 7 bar even though a lower pressure might be adequate in order to provide satisfactory lubrication of the engine components. A high power consumption equates to a reduced vehicle fuel economy, which is a disadvantage.
Another type of known pressure relief valve which is less demanding of the oil pump and therefore can give a better fuel economy is a pilot pressure relief valve. Valves of this type are described in U.S. Pat. No. 6,186,750, for example. In a similar fashion to the above-mentioned conventional relief valve, a piston, biased by a spring moves under a force provided by circulating oil. However, with the pilot-operated valve, the pilot oil pressure applied to the pilot inlet port is taken from a point inside the engine rather than the pump's outlet. Typically, a valve of this type is set to open at 4 bar. Hence, the pump outlet pressure has no effect on the valve's movement. Instead, the valve is operated by the pressure from the engine's oil gallery.
The pilot-operated valve has some disadvantages compared with the conventional one. For example, the start-up oil pressure spike is higher. The reason for this is the time delay between engine start and the pressure rise in the pilot oil feed. For engines having a cam profile switching system, this is especially problematic because the spike can cause an unwanted switch between low lift and high lift tappets. Additionally, higher amplitude pressure pulsations tend to occur at the pump outlet. These pulsations can damage engine components and are due to the time lag between changes in pump outlet pressure and pilot pressure. Further, lubrication systems incorporating pilot-operated valves also require an over-pressure relief valve at the pump outlet in order to protect the filter element. Hence, complexity and cost is added to the lubricating system.
Hence, a pressure relief valve which mitigates at least some of the above-mentioned disadvantages would be advantageous.
According to the present invention, a pressure relief valve for a lubrication system comprises a housing having a bore, first and second ends, an inlet port and an outlet port for, respectively, admitting lubricant from a high pressure side of the lubrication system into the bore and expelling it therefrom, a pilot pressure inlet port at its first end for admitting lubricant taken from a point remote from said high pressure side, and a biasing member and a piston assembly located within the bore, the piston assembly comprising a second piston biased against the first end of the housing by the biasing member and the second piston connected to a first piston and having a smaller diameter than the first piston, wherein a face of the first piston is exposed to pressure from lubricant entering the bore through the inlet port and a face of the second piston is exposed to pressure from lubricant entering the bore through the pilot pressure inlet port.
The pressure relief valve may be an integral part of an oil pump or may be a separate unit.
Compared with the known pilot-operated valve, the pressure relief valve according to the invention will open at the pre-set pressure even if the pilot oil feed has not yet seen the oil pressure, such as at initial engine start-up. Hence there is no requirement for an over-pressure relief valve. Also, engine start-up spikes and pressure pulsations are much reduced and comparable with those experienced with the conventional valve.
Under normal operating conditions, the valve according to the invention is able to keep gallery oil pressure as low as possible, thereby retaining the fuel economy benefits of the known pilot-operated valve.